For liquid crystal displays, backlight units of the edge light type or direct light type are conventionally used. Since backlight units of the edge light type can be manufactured with a small thickness, they are used for notebook computers etc., whereas backlight units of the direct type are used for large-sized LCD television, etc.
Light emitted from these conventional backlight units contains components emitted in directions significantly angled from the front direction, especially light emitted from backlight units of the edge light type, and thus it is difficult to obtain high front luminance.
Therefore, in the conventional backlight units, two or more optical films such as prism sheets and light diffusing films are used in combination in order to improve front luminance so that the light is directed to the front (see, for example, Japanese Patent Unexamined Publication (KOKAI) No. 9-127314 (claim 1, paragraph 0034)).
Although prism sheets can increase the portion of the light emitted to the front (in a direction perpendicular to film surface) by surface design based on geometric optics. However, they have drawbacks in that they are likely to exhibit an interference pattern due to regularly arranged convex portions, and in that, if used alone, they produce glare and thus impair the visibility of images. Moreover, they unduly concentrate light to the front, and therefore they cannot provide a wide viewing angle.
On the other hand, if diffusion films are used alone, the front luminance becomes insufficient, although the aforementioned problems are not caused.
Therefore, a prism sheet and a light diffusing film are used in combination as described above. However, the front luminance enhanced by the prism sheet is reduced by the light diffusing film. Moreover, the films assembled in layers may generate Newton rings between the layers, or become scratched due to their contact.
See Japanese Patent Unexamined Publication (KOKAI) No. 9-127314